This invention relates to a method of determining the position of a mobile unit comprising a GPS receiver.
It is well known to provide a GPS receiver in which replica GPS satellite pseudorandom noise (PRN) code signals are continuously generated and correlated with received GPS signals in order to acquire them. Typically, as the replica codes are likely to have a different code phase to those of the received GPS signals and also a different frequency due to Doppler shift between the receiver and orbiting satellites, a two dimensional code frequency/phase sweep is employed whereby such a sweep will eventually result in the incoming PRN code having the same frequency and code phase as that of the locally generated replica. If detected, the code is acquired and tracked, and the pseudorange information may be retrieved from which the position of the receiver may be determined. Position determination is typically done by conventional but complex navigation computation involves the resolving of at least 4 non-linear equations using an iterative technique such as linearisation. For example, see section 2.4.2 of chapter 2 xe2x80x9cFundamentals of Satellite Navigationxe2x80x9d of GPS Principles and Applications (Editor, Kaplan) ISBN 0-89006-793-7 published by Artech House.
It is further known to provide a mobile cellular telephone incorporating such a GPS receiver for the purpose of enabling operators of cellular telephone networks to determine the location from which a call is made and, in particular, for an emergency call to the emergency services. Of course for an emergency call, it is desirable for the call location to be available as soon as possible, however, from a xe2x80x9ccold startxe2x80x9d where the GPS receiver does not have access to up to date ephemeris data or even worse from a xe2x80x9cfactory cold startxe2x80x9d where the GPS receiver does not have an up to date almanac, the time to first fix (TTFF) can be anywhere between 30 seconds and 5 minutes.
In order to reduce the TTFF, a GPS receiver may be provided with base station assistance in order to acquire GPS signals more quickly. Such assistance may include the provision by the base station to the receiver of a precision carrier frequency reference signal for calibrating the local oscillator used in the GPS receiver and, as obtained by the base station, the data message for up to date satellite almanac and ephemeris data from which Doppler shift for satellites in view can be determined together with the current PRN code phase. With such assistance, it is possible to sweep only a narrowed range of frequencies and code phases in which the GPS PRN code is known to occupy, thereby reducing the number of code instances that need to be checked and thus reducing the time for code acquisition. Base station assistance is further described in U.S. Pat. Nos. 5,841,396 and 5,874,914 which are incorporated herein by reference.
Whilst such base station assistance enables the GPS spread spectrum signals to be acquired more quickly (and hence obtain pseudorange measurements from the GPS satellites to the GPS receiver more quickly), it does not in any way simplify the complex navigation computation required in the GPS receiver to resolve the measured pseudoranges in order to return a position fix. Accordingly, such GPS receivers still require complex and power hungry processors to perform the necessary navigation computation. Furthermore, with respect to GPS receivers incorporated in mobile cellular telephones, power consumption is a particularly acute problem given the fashion for the miniaturisation of mobile telephones (and their battery packs) for both aesthetic and ergonomic reasons.
It is therefore an object of the present invention to provide an improved method of determining the position of a mobile unit having a GPS receiver in which the necessary navigation computation and associated power consumption in the mobile unit is reduced.
According to a first aspect of the present invention, such a method is provided comprising the steps of measuring mobile unit pseudoranges at the mobile unit using the GPS receiver; providing reference pseudoranges corresponding to a reference location situated remote from the mobile unit; and determining the relative position of the mobile unit, i.e. relative to the reference location, as a function of both the reference pseudoranges and the mobile unit pseudoranges. This may be, for example, as a function of the difference between the reference pseudoranges and the mobile unit pseudoranges.
From the reference location and the relative position of the mobile unit, the absolute position of the mobile unit may also be determined.
To further simplify the necessary navigation computation, the method may further comprise the step of providing GPS satellite position information relative to the reference location wherein the relative position of the mobile unit is further determined as a function of the GPS satellite position information. For example, the GPS satellite position information may be provided in the form of normalised direction vectors of the GPS satellites relative to the reference location.
In one preferred method, the position of the mobile unit (Xm, Ym, Zm) relative to the reference location (Xref, Yref, Zref) is determined using the following approximation:                               (                                                                                          X                    m                                    -                                      X                    ref                                                                                                                                            Y                    m                                    -                                      Y                    ref                                                                                                                                            Z                    m                                    -                                      Z                    ref                                                                                                                                            c                    m                                    -                                      c                    ref                                                                                )                ≈                              H                          -              1                                ⁡                      (                                                                                                      PSR                      m1                                        -                                          PSR                      ref1                                                                                                                                                              PSR                      m2                                        -                                          PSR                      ref2                                                                                                                                                              PSR                      m3                                        -                                          PSR                      ref3                                                                                                                                                              PSR                      m4                                        -                                          PSR                      ref4                                                                                            )                                              [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
where cm and cref are the GPS clock errors at the mobile unit and the reference location respectively; PSRmn and PSRrefn are the mobile unit and reference location pseudoranges respectively; and wherein Hxe2x88x921 is the inverse of matrix H which includes normalised direction vectors of satellites relative to the reference location in the form:                     H        =                  (                                                                                                                X                      1                                        -                                          X                      ref                                                                            R                                          1                      -                      ref                                                                                                                                                              Y                      1                                        -                                          Y                      ref                                                                            R                                          1                      -                      ref                                                                                                                                                              Z                      1                                        -                                          Z                      ref                                                                            R                                          1                      -                      ref                                                                                                  1                                                                                                                                X                      2                                        -                                          X                      ref                                                                            R                                          2                      -                      ref                                                                                                                                                              Y                      2                                        -                                          Y                      ref                                                                            R                                          2                      -                      ref                                                                                                                                                              Z                      2                                        -                                          Z                      ref                                                                            R                                          2                      -                      ref                                                                                                  1                                                                                                                                X                      3                                        -                                          X                      ref                                                                            R                                          3                      -                      ref                                                                                                                                                              Y                      3                                        -                                          Y                      ref                                                                            R                                          3                      -                      ref                                                                                                                                                              Z                      3                                        -                                          Z                      ref                                                                            R                                          3                      -                      ref                                                                                                  1                                                                                                                                X                      4                                        -                                          X                      ref                                                                            R                                          4                      -                      ref                                                                                                                                                              Y                      4                                        -                                          Y                      ref                                                                            R                                          4                      -                      ref                                                                                                                                                              Z                      4                                        -                                          Z                      ref                                                                            R                                          4                      -                      ref                                                                                                  1                                              )                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          2                ]            
where Ri-ref is the range between the ith satellite and the reference location.
The reference pseudoranges and/or the GPS satellite position information may be provided using a GPS receiver located at the reference location. This has the advantage that the resultant position fix incorporates the advantages of differential GPS (DGPS) in that errors which, for example, a standard position service (SPS) NAVSTAR GPS could not compensate for such as selective availability, atmospheric effects and perhaps relativistic effects can indeed be compensated for.
As an alternative to providing the reference pseudoranges and/or the GPS satellite position information by using a GPS receiver located at the reference location, they may be provided by calculation based on the reference location and GPS satellite signal information obtained from a GPS receiver located remote from the reference location. Where this is the case, the clock offset at the reference location would of course be a notional offset given that the GPS receiver is not in fact located at the reference location.
The mobile unit pseudoranges may be transmitted from the mobile unit to a communications base station such that the relative position of the mobile unit is determined remote from the mobile unit.
Alternatively, the pseudoranges may be transmitted from a communications base station to the mobile unit and the relative position of the mobile unit determined at the mobile unit. In this case, the relative position of the mobile unit may be transmitted from the mobile unit to the communications base station. Also, the reference location may be transmitted from the communications base station to the mobile unit and the absolute position of the mobile unit determined from both the reference location and the relative position of the mobile unit. The absolute position of the mobile unit may then be transmitted back to the communications base station.
For convenience, the reference location may geographically coincide with the location of the communications base station, for example, where a GPS receiver for providing reference pseudoranges is located at the communications base station.
Also provided in accordance with the first aspect of the present invention is a further method of determining the position of a mobile unit having a GPS receiver comprising the steps of measuring pseudoranges at the mobile unit; determining the position of the mobile unit relative to a nearby reference location using the pseudoranges; and determining the absolute position of the mobile unit from both the reference location and the relative position of the mobile unit to the reference location.
Where NAVSTAR GPS is concerned, the ephemeris data is provided as a set of Keplerian parameters (with added perturbation terms) whereas in order to simplify the navigation computation such as in accordance with equation 2 above, it is necessary to express the direction of the satellites relative to the reference location in vector form. As such, and in accordance with a second aspect of the present invention, a further method of determining the position of a mobile unit having a GPS receiver is provided comprises the steps of measuring mobile unit pseudoranges at the mobile unit using the GPS receiver; receiving GPS ephemeris data in the form of Keplerian parameters at a reference location located remote from the mobile unit; calculating from the ephemeris data satellite vector information in Cartesian form describing either or both of the position or movement of GPS satellites; providing the satellite vector information to the mobile unit; and determining the position of the mobile unit as a function of the satellite vector information and the mobile unit pseudoranges.
The satellite vector information may comprise position (x, y, z), velocity ({dot over (x)},{dot over (y)},{dot over (z)}), acceleration ({umlaut over (x)},ÿ,{umlaut over (z)}), 4th (i.e. jerk) and higher order vectors, each describing either the position or movement of the GPS satellites.